1. Field of the Invention
The invention generally relates to the evaluation of relative flatness of transparent or reflecting sheet material and more particularly to an inspection system for measuring the flatness quality of glass sheets by a uniform standard in a logic indicative of the surface flatness for comparison to a like logic for a prescribed surface flatness.
2. Description of the Prior Art
In the past, visual methods have been employed in evaluating the flatness quality of planar glass surfaces including the use of a device commonly known as an "optical flat". As is known, an optical flat is a tool for the optical inspection of the flatness of a surface. The tool consists of a polished piece of clear glass ground to have flat parallel surfaces, which is placed on the surface to be inspected. When the glass is illuminated with a monochromatic light, variations in the direction and spacing of light bands (known as the interference pattern), are produced by the light reflected from the surface being inspected and that reflected from the lower surface of the optical flat. The interference pattern will reveal any variations in the thickness of the film of air between the inspected surface and the optical flat. The interference pattern may be composed of any number of light and dark bands known as fringes. The utility of such a method as an ultimate tool in determining flatness quality of glass sheets is limited, however, in that human judgment is involved in determining whether a particular sheet of glass falls within established flatness quality limits. One factor which limits such a visual inspection system is the resolution ability of the human eye. Also, such devices are not satisfactory for factory operation on a production basis in that they tend to be susceptible to dirt on them and/or the glass being inspected, which consequently compounds an erroneous visual reading due to the human element. In order to permit establishment of uniform standards for the grading of glass into flatness categories suitable for different end uses independent of visual inspection and its attendant human variation, a procedure which eliminates the visual inspection of planar glass for flatness has long been sought.
Heretofore, inspection systems such as shown in assignee's U.S. Pat. No. 3,788,750, issued on Jan. 29, 1975, have been used to detect vision distortion or optical quality by observing the lens power and width of defects which appear as positive and negative lenses on the surfaces of a reflecting sheet material. As shown therein, components similar to the type used in the instant invention are employed to observe conditions which cause visual distortion in flat glass sheets occurring as longitudinal streaks, thickness variations, or non-planar areas thereon.